A mass spectral study of stable neutral enols

The high‐resolution mass spectra of two stable enol tautomers, 4,6‐diethanoyl‐1‐methyl‐5‐phenyl‐3‐cyclohex‐ene‐1,3‐diol (2) and 4‐ethanoyl‐5‐hydroxy‐2‐(1‐hydroxyethylidene)‐5‐methyl‐3‐phenylcyclohexanone (3), and the corresponding diketone tautomer, 2,4‐diethanoyl‐5‐hydroxy‐5‐methyl‐3‐phenylcyclohexanone (1), are reported. Collisional activation of the molecular ions and major ionic fragments is used to assign decomposition pathways and to compare ion structures. Ionized 1 and 2 do not interconvert by a keto‐enol tautomerism, but isomerize to a common intermediate. This isomerization probably involves opening of the cyclohexanone ring and transfer of the enolizable hydrogen, and is followed by elimination of water. The radical cation of 3 does not interconvert with [1]⁺˙ and [2]⁺˙ Instead, [3]⁺˙ dissociates by loss of an acetyl radical and eliminates water via a higher‐energy pathway. As a result, the mass spectra of 1 and 2 are similar, whereas the mass spectrum of 3 is unique.